BMS 3.5" vs 5" downpipes

[Xcution]

Hey wanted some wisdom on what are the pros and cons of each of the downpipes here. Example more hp/tq, weights less, etc. I am planning on buying one with JB4 and want to make the best choice for me.

Links:

BMS N55 Downpipe 135i 3.5":

http://burgertuning.com/N55_BMS_BMW_...downpipes.html


cpe-e BMW 135i 5":

http://burgertuning.com/CPE_N55_BMW_..._downpipe.html

Thanks for all the help!

[Everfly]

They both said gain 20hp 30 torque from the website.

[Xcution]

Exactly so what is the different. The larger the diameter the better the flow. If they provide the "same" hp is the only difference look and/or sound???

Anyone purchase one of these and can talk more to it. Hopefully we can get some input on both.

thanks,

[iNAN3 SOLDi3R]

Subscribing...

I am in the same spot. Since they both claim the same hp/tq #'s, the weight and sound is a key factor for me. I can't stand drone...

[That_1_Guy]

Originally Posted by Xcution

Exactly so what is the different. The larger the diameter the better the flow. If they provide the "same" hp is the only difference look and/or sound???

Anyone purchase one of these and can talk more to it. Hopefully we can get some input on both.

thanks,

I was wondering the same thing a while back and tried bringing it on here, but the conversation did nothing but go back and forth. One person says 5" is too much and it will lag, but then someone else says the bigger the better. One person claims 3.5 to 4" is the sweet spot, while some would put their life on a 5".

When it boiled down to it, I messaged Terry@BMS, and I would put all my trust in that guy when it came to modding E8x's.

Terry said bigger is better. 5" will give more flow and a better sound, along with a bit of a better power gain.


I am sure people will have the lag argument, or the price argument, but when it really boils down to it, if you can afford the 5" cp-e DP (which someone on the 3 forums market place is selling for 499), I would buy it. I just bought a brand new AR dp for a steal of a deal, or I would be all over it!



Just my 2 cents

[Mr Kleen]

With stock turbo(s) there is no difference.

[splitsecond]

There was a thread on n54tech where some guy asked Terry (the BMS guy) about the pros and cons of switching from his AA downpipe (which tapers to 2.5" at the bottom) to a 3.5" BMS downpipe (which was brand new at the time). Terry advised the guy not to waste his money because he didn't think there was much of a difference in performance. So if Terry would say that there's no big advantage of 3.5" over 2.5" (even though the 3.5" was his own product), then I'd imagine that the difference between 5" and 3.5" would be even less.

[Stohlen]

The 5 inch piping is in my opinion too big for the stock turbo system. Even 1000hp supra's use 4 inch piping all the way back. That large opening where the two pipes connect will drastically reduce exhaust velocity and cause a bottleneck where the 5 inch piping tapers down to meet the exhaust.

The exhuast then has to increase in velocity suddenly because of the increase in pressure at the bottleneck. However because it has been slowed, all momentum of the exhaust gas has been lost. Its much like missing a shift in a drag race. There is a momentary loss of all power and then you have to take extra time to recover from it to continue on.

This slowing of the exhaust gases stops the suction-like effect of those exhaust gases. Instead of creating a high speed vacuum that pulls the exhaust gases through the turbo, pressure builds up and the gases must be pushed through from the opposite side, which creates more heat, and results in an air clot at the exhaust valve. This air clot slows the flow of exhuast out of the cylinder which allows less air to be forced inside the cylinder, leading to a less efficient engine, and less peak horsepower at WOT.

Ideally you want a consistant pipe diameter from the turbo on back to eliminate any slowing of the exhaust gases that can lead to air clots and a loss of peak power. On a traditional 3.0L Turbocharged engine, a 3"-3.5" single pipe is ideal and a 2.25"-2.75" pipe is ideal when running two pipes. For most people on this forum, getting a 3.5" exhuast with 2.5" pipes running to it is a good happy medium to reduce back pressure while keeping as much exhaust velocity as possible to aid in cylinder exhaust removal. This size will also allow you to make modifications to your engine (IE bigger turbos, methanol, e85, etc.) that add more exhaust gas to the system. In other words, a 3.5" exhaust should be good till 500-600whp without causing massive amounts of back pressure.

[Stohlen]

Remember, bigger isn't always better, especially not when it comes to exhaust piping.

[Freon]

On a turbo car, you pretty much never lose by using larger piping after the turbo. Any pulses are destroyed at the turbine. You do not need any scavenging effect. The turbine works best with the absolute least pressure after the turbo possible.

That said, 5" is way overkill. Yes, 1000hp Supras are using 4", which is probably fine for that power level. It gets tapered to 2*2.75" anyway by the end of the downpipe so I don't think having it expand to 5" diameter for a short length is really going to help much.

I don't think it really matters between these two designs. I do like the cast parts on the CP-E more than the tubular BMS, as the transistions look slightly smoother, but you pay a bit extra for that. Again, I doubt it is really going to translate to any measurable power difference.

The biggest gain here is removing the cats. I wouldn't worry about it that much beyond that. Queue this thread becoming a pissing content because we'll never have good data...